1. Field of the Invention
This invention relates in general to mercury vapor discharge lamps and pertains, more particularly, to a fluorescent lamp which contains an agent for substantially reducing the amount of mercury which is extractable after pulverization of the lamp.
2. Description of the Prior Art
Fluorescent lamps are well known in the art and are characterized as mercury vapor discharge lamps which include a pair of electrodes sealed in an elongated envelope whose interior surface is coated with phosphor. The envelope contains a quantity of mercury and a rare gas at a reduced pressure, for example, in the order of 1-5 torr.
During lamp manufacture, about 5-40 milligrams of elemental mercury is sealed in each fluorescent lamp. It is known that most of the mercury adheres to the phosphor coating with only a very small portion of the mercury being in the form of mercury vapor. After alkaline earth metal oxides coating the lamp electrodes are volatized, the oxides decompose in the discharge space and freed oxygen converts some of the elemental mercury to a salt or compound, such as mercuric oxide (HgO) which is highly soluble.
There is concern that a waste stream resulting from the disposal of fluorescent lamps may leach excessive amounts of the soluble form of mercury. One method of measuring the amount of soluble mercury which may leach from the waste stream resulting from the disposal of fluorescent lamps is described in the Toxicity Characteristic Leaching Procedure (TCLP) prescribed on pages 26987-26998 of volume 55, number 126 of the Jun. 29, 1990 issue of the Federal Register. According to the procedure, the lamp being tested is pulverized into granules having a surface area per gram of material equal to or greater than 3.1 cm.sup.2 or having a particle size smaller than 1 cm in its narrowest dimension. Following pulverization, the granules are subjected to an extraction fluid comprising a sodium acetate buffer solution having a pH of approximately 4.93 and having a weight twenty times the weight of the granules.
The Environmental Protection Agency (EPA) defines a maximum concentration level for mercury at 0.2 milligram extractable mercury per liter of extract fluid when the TCLP is applied. Accordingly, it is desirable that the concentration of mercury dissolved in the extraction fluid be as low as possible, and particularly less than 0.2 mg per liter of extraction fluid.
Various methods have been proposed which attempt to treat or process burned-out discharge lamps, or scrap lamp exhaust tubing containing mercury, in order to reclaim the mercury and thereby reduce the amount of mercury-contaminated scrap glass. For example, U.S. Pat. No. 4,268,306 (Bjorkman) discloses a treatment whereby the expended or scrapped lamps are crushed into fragments which are then placed in a treatment vessel in which the mercury is recovered by distillation processes. This treatment is carried out batchwise in a sealed container into which nitrogen is introduced. The container is heated and placed under vacuum to vaporize the mercury. The vaporized mercury is then removed from the container and condensed in a cooling trap. Following the process, the treated lamp fragments, i.e., glass and phosphor, may be dumped without concern about mercury leaching.
U.S. Pat. No. 4,715,838 (Kulander) teaches a method and apparatus for recovering the luminescent material from mercury vapor electric discharge lamps, whereby the amount of lamp waste requiring treatment in a distillation chamber can be reduced, thereby increasing the capacity of the chamber with respect to the number of lamps. After the ends of the lamp bulb, or tube, are separated from the intermediate bulb part, the luminescent material is loosened from the inner wall surface of the bulb part with the aid of a stripping device which is arranged to be inserted into the bulb part from one end thereof. The loosened material is collected with the aid of a suction device connected to the other end of the bulb part and, thereafter, may be introduced into the distillation chamber, thereby obviating the need to fill the chamber with the residual "clean" glass bulb.
Japanese Patent Application No. 58-184558 (Kitsugi et. al.) teaches a method for solidifying discarded fluorescent lamps and insolubilizing the mercury contained therein. The method involves adding sulfur to crushed fluorescent lamp waste. The result is pulverized an amount of time sufficient to produce granules with sizes 5 millimeters or less. Cement and water are added and the result is kneaded. The kneaded material prepared in this way is put into molds to form a stabilized solidified material having a mercury concentration of less than 0.005 milligram per liter water.
U.S. Pat. No. 4,435,284 (Heytmeijer) teaches a process for removing residual mercury from scrap fluorescent lamp exhaust tubulation, or similar scrap glass. A very small predetermined amount of finely divided silica or alumina powder is added to the mercury contaminated scrap glass and the mixture is agitated for a short predetermined time during which the mercury is stripped from the tubulation. The separated mercury, powder and glass mixture is then decanted through a screen of suitable mesh to remove the separated mercury and powder from the glass. The mercury is then rinsed with a suitable solvent to remove the oxide powder from the mercury.
U.S. Pat. Nos. 5,229,686 (Fowler et. al.) and 5,229,687 (Fowler et. al.) relate to a mercury discharge lamp which can be disposed of without prior expensive treatment to reclaim mercury. Such is accomplished by the provision of a mercury vapor discharge lamp having an envelope of light-transmitting vitreous material containing an inert starting gas and a quantity of elemental mercury at least partially convertible to soluble mercury. In the '686 patent, an effective amount of a chemical agent suitable for chemically converting a substantial portion of the mercury to a sparingly soluble salt when the lamp is pulverized to granules and subjected to a suitable aqueous acid solution is located within the lamp. Preferably, the sparingly soluble salt is soluble to less than 0.2 milligram per liter mercury. In a preferred embodiment, the chemical agent comprises approximately 3.45 grams of potassium periodate. The lamp further includes a sealed enclosure (e.g., glass) for containing the chemical agent. The sealed enclosure is disposed within the lamp envelope or external to the envelope, such as within a cavity defined by a lamp base member. The sealed enclosure is rupturable upon pulverization of the lamp. In an alternative embodiment, the chemical agent is mixed with the basing cement used to secure one or more of the lamp bases to the lamp envelope. Unfortunately, the quantity of chemical agent required to chemically combine nearly all of the mercury within a fluorescent lamp may be so large as to be inconvenient or impossible to contain within a standard lamp envelope.
In the '687 patent, first and second electrodes are located within the lamp envelope for establishing an arc discharge therebetween. An effective amount of a chemical agent suitable for electrochemically reducing a substantial portion of the soluble mercury to elemental mercury when the lamp is pulverized to granules and subjected to a suitable aqueous acid solution is located within the lamp and effectively remote from the arc discharge. The chemical agent is an element having an electrode potential for oxidation reactions higher than mercury but not sufficiently active to displace hydrogen from the aqueous acid solution. In one embodiment, the chemical agent comprises approximately 0.5 to 5 grams of copper powder or copper dust. In another embodiment, the chemical agent comprises approximately 0.25 gram to 5 grams of iron powder. The lamp further includes a sealed enclosure (e.g., glass) for containing the chemical agent. The sealed enclosure is disposed within the lamp envelope or external to the envelope, such as within a cavity defined by a lamp base member. The sealed enclosure is rupturable upon pulverization of the lamp. In an alternative embodiment, the chemical agent is mixed with the basing cement used to secure one or more of the lamp bases to the lamp envelope. Again, however, it may be difficult or impossible to incorporate a sufficiently large quantity of a finely divided metal within a fluorescent lamp, particularly in smaller and more compact lamps. Further, if not completely encapsulated, the metallic substance may interfere with or even prevent the normal operation of the electric lamp.
Accordingly, there is a need for a fluorescent lamp having disposed in the envelope thereof an agent which in small quantities is effective to reduce the amount of extractable mercury within the lamp to less than 0.2 milligram per liter of an aqueous acid solution, as prescribed in the TCLP.
There is further a need for a method for reducing mercury that is extracted from a fluorescent lamp to less than 0.2 milligrams per liter of an aqueous acid solution, as prescribed in the TCLP.